The invention is directed towards the field of lasers, and more specifically, towards alloys that can be used in the active region of a laser.
Vertical cavity surface emitting lasers (VCSELs) are commonly used as light sources in optical communication systems. FIG. 1A shows a diagram of a prior art VCSEL 101, based on a gallium arsenide (GaAs) substrate 102. VCSEL 101 emits light at 850 nm. Two mirror stacks 103, one adjacent to the substrate 102 and one at the top of the VCSEL 101, reflect the laser light within the VCSEL 101. The mirror stacks 103 are typically Distributed Bragg Reflectors (DBRs) made of alternating layers of AlxGa1-xAs and AlyGa1-yAs, where xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d denote the molecular fractions of Al in high and low refractive index layers, respectively. A cladding layer 107 is adjacent to each mirror stack 103. Although each cladding layer 107 is illustrated as a single layer, it may be composed of many different layers. The cladding layer 107 may also be called a spacer, and is used to pad the size of an active region 109 so that the VCSEL 101 will work properly. Sandwiched between the mirror stacks 103 and cladding layers 107 is the active region 109, comprising interleaved layers of quantum wells 111 and barrier layers 113. The quantum wells 111 have a width w. The quantum wells 111 are typically GaAs, and the barrier layers 113 are typically AlGaAs. Hereinafter, VCSELs shall be referred to by the composition of their active region. Therefore, the VCSEL 101 can be identified as a GaAs/AlGaAs VCSEL, or alternatively as a VCSEL with a GaAs/AlGaAs active region.
FIG. 1B shows an energy-band diagram identifying selected band parameters for the active region 109 of the VCSEL 101 shown in FIG. 1A. The conduction band is labeled Ec and the valence band is labeled Ev. The difference between the conduction band Ec and the valence band Ev is known as a band gap. The band gap of the quantum well 111 is labeled EgQW. The band gap of the barrier layer 113 is labeled EgB. The difference between the conduction bands Ec of the quantum well 111 and the barrier layer 113 is known as the conduction band offset, labeled xcex94Ec. The difference between the valence bands Ev of the quantum well 111 and the barrier layer 113 is known as the valence band offset, labeled xcex94Ev. Electrons and holes (collectively known as carriers) are injected into the quantum well 111 and confined by the barrier layers 113 when the VCSEL is forward biased. Ideally, the materials used in the quantum wells 111 and barrier layers 113 have a relatively large xcex94Ec and xcex94Ev to provide effective carrier confinement in the quantum well 111. In a typical GaAs/AlGaAs VCSEL 101, xcex94Ec≈150 meV and xcex94Ev≈75 meV. Note that xcex94Ec is twice xcex94Ev; a 2:1 ratio between xcex94Ec and xcex94Ev is often considered indicative of a well-balanced material system.
Carriers inside the quantum well 111 actually acquire a slight amount of energy as a result of their confinement, effectively increasing the quantum well bandgap EgQW by the energy of quantum confinement dEqc (not shown). dEqc is a function of the quantum well width w, increasing as w is decreased. When the active region 109 is not lattice-matched to the substrate 102, the carriers within the quantum well acquire an additional energy due to lattice strain dEstrain (not shown). Although the band parameters described above refer specifically to the active region 109 of the VCSEL 101, the terms are equally applicable to the active region of any laser.
Light is emitted from the quantum well 111 when electrons drop from the conduction band Ec to the valence band Ev. The wavelength of light emitted is determined approximately by the formula:                               λ          um                ≈                              1.24            ⁢                          xe2x80x83                        ⁢            eV                                              E              g              QW                        +                          dE              qc                        +                          dE              strain                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
In Equation 1, EgQW is the greatest contributing factor in determining the wavelength, as it is typically much larger than dEqc or dEstrain. The material used for the quantum well 111 should be selected to have a band gap EgQW that will produce light within the desired range of wavelengths. The quantum well width w and lattice strain on the substrate 102 will also be a consideration because of dEqc and dEstrain.
GaAs/AlGaAs is ideal for the active region in a GaAs-substrate VCSEL for several reasons. First, (Al)GaAs/AlGaAs can be used to implement both the mirror stacks 103 and the active region 109, thus simplifying the manufacturing process because there is no need to change the growth conditions. Second, mirror stacks 103 using AlGaAs/AlGaAs can be epitaxially grown on the GaAs substrate 102, which results in a VCSEL that is entirely grown epitaxially. Since fully-epitaxial VCSELS are easier to manufacture and process, they are preferred over VCSELS having mirror stacks formed with other methods such as fusion bonding or deposition. Third, GaAs/AlGaAs VCSELs can be oxidized. Oxidized layers are often used in a VCSEL to electrically confine carriers and optically confine the laser beam, which leads to improved electro-optical performance of the device.
One final reason that GaAs/AlGaAs VCSELs work well is due to their low sensitivity to temperature. A VCSEL typically has to maintain performance within an operating temperature range between 0-100xc2x0 C. One parameter used to measure temperature sensitivity is known as the characteristic temperature T0. T0 is usually determined for broad area lasers (also known as edge-emitting lasers), not for VCSELs. However, the T0 of an edge-emitting laser built with a given active region is still a useful indicator of how that same active region will perform with temperature changes in a VCSEL. A high characteristic temperature T0 is preferable because it means the laser is less sensitive to temperature fluctuations. An edge-emitting laser built with a GaAs/AlGaAs active region typically has a characteristic temperature T0 around 150K, which is relatively high. The characteristic temperature T0 is also related to xcex94Ec and xcex94Evxe2x80x94an active region with large xcex94Ec and xcex94Ev will likely exhibit high T0 and low threshold current density, provided that the material quality is good.
The light emitted from a GaAs/AlGaAs VCSEL typically has a wavelength around 850 nm, which has a transmission range of 200-500 m in multimode fiber, depending on the speed of the optical link. Currently, the challenge facing the optical communications industry is creating a VCSEL capable of emitting light with a longer wavelength, which can travel longer distances along a single-mode optical fiber. The preferable target wavelength range is between 1.2 um and 1.4 um, or more specifically, 1260-1360 nm, which would produce transmission ranges of 2-40 km. The ideal long-wavelength VCSEL would possess the same qualities as a GaAs/AlGaAs VCSEL (i.e. epitaxially grown mirrors, active regions that are lattice matched to the substrate, good carrier containment, low sensitivity to temperature changes, etc) except with a longer wavelength of emitted light.
Several material systems have been proposed that would emit light within the target range. One approach is using InGaAsN/GaAs or InGaAsN/GaAsN (hereinafter collectively referred to as InGaAsN/GaAs(N)) in the active region on a GaAs substrate. InGaAsN/GaAs(N) has acceptable performance over the desired temperature range. Unfortunately, although InGaAsN/GaAs(N) can be epitaxially grown on the GaAs substrate, the lattice structure does not match well to the GaAs substrate and introduces a compressive strain of 3% or more. Such a large strain may cause undesirable reliability problems in a VCSEL.
Another approach to long-wavelength VCSELs involves using a substrate of indium phosphide (InP). InP has been researched extensively as a VCSEL substrate, and many materials have been identified that can form epitaxially-grown mirror stacks on InP. For example, InGaAsP/InP was a promising material system for VCSELs, since InGaAsP can be lattice-matched to the InP substrate and epitaxially grown to create mirror stacks. However, the small conduction band offset (xcex94Ec) in an InGaAsP/InP active region does not allow for effective electron confinement at elevated temperatures. Since VCSELs must operate over a wide range of temperatures up to 100xc2x0 C., the InGaAsP/InP material system is not an ideal solution. Another drawback to the InGaAsP/InP material system is that it cannot be oxidized to create the desired optical and electrical confinement within the VCSEL.
AlInGaAs/AlInGaAs active regions grown on InP have also been investigated. However, the characteristic temperature of edge-emitting lasers made with AlInGaAs/AlInGaAs is only in the range of 100-120K. A higher characteristic temperature would be preferable to minimize the VCSEL""s sensitivity to temperature changes. This is especially important since the thermal conductivity of epitaxial mirrors grown on InP substrates is known to be low.
Therefore, a need remains for a VCSEL long-wavelength material system that has relatively large band offsets (xcex94Ec and xcex94Ev) for effective carrier confinement within the quantum wells in the temperature range of interest, with a lattice structure that substantially matches the substrate""s lattice structure, and a relatively high characteristic temperature. Preferably, the mirror stacks of the VCSEL can be epitaxially grown on the substrate. It would also be preferable that the mirrors can be oxidized, since oxidation is an effective way to provide electrical and optical confinement of currents and optical beams.
In accordance with an illustrated preferred embodiment of the present invention, a VCSEL based on an InP substrate is disclosed. The VCSEL has a first mirror stack and second mirror stack, the first mirror stack adjacent to the substrate. Sandwiched between the two mirror stacks are two cladding layers. Sandwiched between the two cladding layers is an active region. The mirror stacks are preferably grown epitaxially, although other methods of fabricating the mirror stacks are acceptable. The active region uses AlGaAsSb quantum wells interleaved with barrier layers of AlGaAsSb, AlInGaAs, or AlInAs. Alternatively, the active region of the VCSEL can comprise quantum wells of GaAsSb interleaved with barrier layers of AlGaAsSb, AlInGaAs, or AlInAs.
As will be discussed in the following section, the present invention advantageously provides an active region that not only emits light within the desired long-wavelength range, but also has a substrate-matching lattice structure. Furthermore, the present invention provides effective carrier containment over the entire operating temperature range. This is a combination of features not available in the prior art.
Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.